1. Field of the Invention
This invention relates generally to the field aids to air navigation and more specifically to the use of such a frequency tracking filter in a radio directional guidance system based on the Doppler principle.
2. Description of the Prior Art
Doppler radio navigation systems have been extensively described in the technical literature. For example, the technical journal, "Electrical Communication", published by International Telephone and Telegraph Corporation, Vol. 46 (1971), Number 4, contains an extensive article entitled "Doppler Scanning Guidance System". Also, the U.S. and European patent literature describes various aspects of such systems. British Pat. Nos. 1,225,190 and 1,234,541 and U.S. Pat. Nos. 3,613,096 and 3,601,684 are also pertinent for an understanding of the state of the prior art in such systems.
Moreover, for an understanding of other approaches and solutions to the multipath reception problem, U.S. Pat. Nos. 3,737,908; 3,774,222; 3,775,773; 3,781,897 and 3,795,002 are of interest.
In the basic Doppler system, a radio frequency source is commutated successively to the separate elements of a linear array of radiators, either to simulate uni-directional constant velocity motion of a single radiator, or an otherwise programmed scan, such as a to-and-fro constant velocity motion.
The angular position of a receiving station remote from the array is defined by the frequency of the commutated wave emitted in that direction, and particularly with azimuth systems using a horizontal array for provision of bearing information over a sector of typically .+-.60.degree. normal to the axis of the array, frequency-counting type receivers used in the system must accept a band of frequencies which covers the complete sector of possible range of frequencies corresponding to such an angular sector.
However, the received bearing information signal does not consist of only the discrete information frequency (by direct propagation) but also of multipath components (by reflection).
In azimuth systems these multipath signals tend to be spread over the sector, in that they are derived from objects which subtend differing angles from the transmitter. Fortunately, however, the multipath spectra are unlikely to be crowded close to the desired information components.